Gas hydrates in sandy reservoirs interpreted from velocity pull up: Are Mississippi-fan turbidites diffusively charged?

Andrew Madof, Chevron

Gas hydrates are recognized as an emerging energy resource and submarine geohazard; they are also thought to be a modulating mechanism on the global organic carbon budget and on past climate change. Although identified primarily from reflectivity changes at the base of the stability zone, gas hydrates located above this boundary are regularly difficult to interpret. Here, I introduce a non-reflectivity travel-time based method to detect gas hydrates in sandy reservoirs. The technique uses seismic travel-time deficits below high-velocity deposits in the stability zone to identify gas hydrate accumulations, and magnitudes of velocity pull up (VPU) to quantify in-situ saturation. The approach has been applied to a portion of the central Gulf of Mexico and has uncovered continuous high-velocity accumulations contained within sandy turbidites of the Quaternary Mississippi fan. Deposits extend more than 175 km southeast, and are interpreted to be vast and previously unidentified gas hydrates locally reaching saturations >70%. Based on reflection character and a marked lack of faulting, accumulations are inferred to have been sourced by short-migration diffusion of gas, making them one of the only known interpreted seismic examples of a non-focused-flow gas hydrate system. Further application of the VPU method can be used to provide insight into gas-migration mechanisms, and to catalogue worldwide distributions of gas hydrates in sandy reservoirs.